FIG. 1 is an illustration of wave 100 undergoing equivalent time sampling. The positive edge of transmit trigger pulse 101 causes a transmitter (not shown) to transmit a signal that returns to the receiver as wave 100. The negative edge of receive trigger pulse 101 causes a receive unit (not shown) to listen to wave 101, thereby sampling it and holding it for a very short window. Each subsequent negative edge of receive trigger pulse 102 is increasingly offset from its corresponding transmit trigger pulse positive edge so that subsequent samples advance over the contour of a cycle of wave 100. Sample and hold output 103 is a reconstructed pulse of wave 101, stretched out in time. The increasing offset of receive trigger pulse 102 affects the resolution—an offset that increases rapidly between transmit/receive pulse pairs will generally “see” less of wave 100, while an offset that increases more slowly will generally see more of wave 100.
Conventional equivalent time sampling systems can be used with time domain radar to effect a time-stretch of the received radar signals, as shown in FIG. 1. For example, if each pulse cycle of wave 100 lasts ten nanoseconds, it may be down-converted by an equivalent time sampling approach and stretched to an equivalent shape in a ten millisecond period. If this down-conversion is accomplished using a linear profile (i.e., the time offset increases linearly for subsequent transmit/receive pairs across the sampled time period), the resolution and fidelity of output 103 will generally be constant across the sampled time period. FIG. 2 shows a linear sampling profile of the prior art.
It should be noted that in radar systems, later parts of a received waveform are generally considered to be reflected from objects that are farther away, such that as the real time axis of FIG. 2 increases, the information in those samples is often associated with greater ranges (and in the case of ground penetrating radar, depths). It should also be noted that attenuation due to ground per unit depth increases as frequency increases and is especially pronounced at frequencies above 1 GHz. Because of this phenomenon, it is more feasible to have higher fidelity and greater Signal to Noise Ratio (SNR) for the near-time (shallow) range. By contrast, for the late time range, higher fidelity is often less feasible, due to the attenuation of higher frequencies at greater depths. Further, deeper objects tend to be larger, such that higher resolution is often unnecessary.
Radar systems typically generate a frame of data during an acquisition window, the acquisition window being measured in equivalent time. Shorter frames generally provide less information by sacrificing resolution and/or range, whereas longer frames decrease the rate at which a radar system can scan an area (i.e., its advance rate), since the number of frames per second is lower. Accordingly, radar designers are often faced with competing parameters—frame size/advance rate and resolution/range.
One way to see at greater ranges using the same frame length is to change the equivalent time sample down-conversion ratio to expand the real time range, thereby covering more real time range in the same equivalent time. This can be accomplished, for example, by using a greater rate of increase for the time offset between subsequent transmit/receive operations. Prior art systems use a single down-conversion ratio (as in FIG. 2), thereby producing the same resolution for all parts of an acquisition window. Therefore, for the same number of samples in a frame, such range expanding lowers the resolution of the output across the frame in prior art systems. It follows that increasing the resolution for the same frame size and number of samples decreases the range of the acquisition window in prior art systems. Currently, there is no system that provides performance at near and far ranges without sacrificing resolution of near range data and without significantly expanding frame sizes.